Calc

An extensible, advanced desk calculator
and mathematical tool written in Emacs Lisp by Dave
Gillespie <daveg@synaptics.com>. Calc runs as part of GNU
Emacs. You can use Calc as only a simple four-function
calculator, but it also provides additional features including
choice of algebraic or RPN (stack-based) entry,
logarithms, trigonometric and financial functions, arbitrary
precision, complex numbers, vectors, matrices, dates, times,
infinities, sets, algebraic simplification, differentiation,
and integration.